La Samanna and Isle de France are very much alike. Both top luxury on world class beaches. I would have to go with long Beach at La Samanna as the beast beach I have ever sat upon.
It is long, sandy and has a drop off that has incredible snorkleing. It has a very private feel to it unlike Flammons which is also an incredible beach. Both St Martin and St Barths have uncomparable dining possibilities. Both have incredible beaches.
St Martin is a blast of cultural color, traffic and crime. St Barths is safer, and homogeneous, like being within the country club gates. A tough choice.